The world’s average temperature has increased 1°C in just the past 100 years. Extreme weather and rising seas are already causing global unrest, and many scientists believe that if we cannot curb planetary warming, it could pose an existential threat to human civilization. As it becomes clear that emissions reductions alone may not be enough to avoid dire effects of climate change, a growing collective of scientists and engineers is developing technologies that could cool the planet—from geoengineering our atmosphere to reflect sunlight, to sucking carbon dioxide right out of the air, to enlisting forests to do the job for us. NOVA joins scientists and skeptics alike to examine the potential—and the peril—of the sometimes-controversial technologies that could help us combat a looming threat.